1. The Field of the Invention
The present invention relates to systems and methods for regulating the flow of a liquid through tanks or reservoirs. More particularly, the present invention relates to a shielding apparatus for preventing the premature flow of a liquid through an overflow drain tube. The shielding apparatus may be used in systems for cleaning semiconductor structures in an acid bath.
2. The Relevant Technology
Integrated circuits and other semiconductor structures are currently manufactured by an elaborate process in which semiconductor devices, insulating films, and patterned conducting films are sequentially constructed in a predetermined arrangement on a semiconductor substrate. In the context of this document, the term "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term "substrate" refers to any supporting structure including but not limited to the semiconductor substrates described above. The term "semiconductor structure" shall refer to any construction that includes semiconductive material or that is formed over a semiconductor substrate. The term semiconductor substrate is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire.
During the process of manufacturing integrated circuits and other semiconductor structures, contaminants, such as particulates and residue, accumulate on surfaces thereof. These contaminants generally must be cleaned from surfaces of semiconductor structures in order to ensure reliable operation of the finished product and to allow formation of further layers and structures on the surfaces. A common method of cleaning semiconductor structures involves an acid bath in which the semiconductor structures are placed in a tank or reservoir containing an acidic solution.
A conventional system for cleaning semiconductor structures is seen in FIG. 1. Cleaning system 10 includes an inner first reservoir 12 and an outer second reservoir 14. A plurality of semiconductor wafers 16 are placed within first reservoir 12, and are commonly secured in position using a boat or another carrier. Cleaning system 10 is operated by circulating a liquid through the reservoirs 12 and 14 using pump 18. The liquid may be, for example, an acidic solution including sulfuric acid.
A first volume 20 of the liquid is contained in first reservoir 12 and a second volume 22 of the liquid is contained in second reservoir 14. Pump 18 causes the liquid to flow through an inlet channel 24 and through an inlet orifice 26 into first reservoir 12. The acidic solution flows over the surfaces of semiconductor wafers 16 in first reservoir 12, thereby dissolving, chemically degrading, or otherwise washing away contaminants and impurities. The liquid leaves first reservoir 12 by flowing or cascading over inner wall 28 and into second reservoir 14. The liquid is held in second reservoir 14 until it is pumped into outlet orifice 30 and through outlet channel 32 into pump 18. During operation of cleaning system 10, the flow of liquid through pump 18 and the elevations of first volume 20 and second volume 22 remain substantially constant.
Occasionally, outlet channel 32 is blocked, or the normal flow of liquid through cleaning system 10 is otherwise disrupted. In order to deal with such situations, a safety mechanism, including overflow drain tube 34, is included in cleaning system 10 to prevent the acidic solution from overflowing into the surrounding environment. This is particularly important because spillage of the acidic solution would be hazardous to nearby technicians, other workers, and laboratory and industrial property.
As seen in FIG. 2, overflow drain tube 34 typically extends through second volume 22 and has a substantially vertical axis through a section 35 that terminates at rim 36 surrounding opening 38. Opening 38 is positioned at a predetermined elevation over the normal operating elevation of second volume 22. Under ordinary conditions, second volume 22 has an equilibrium elevation that is several inches lower than the elevation of first volume 20. When cleaning system 10 malfunctions and second volume 22 rises to an elevation equal to or slightly higher than the predetermined elevation, some of the liquid pours over rim 36 and is removed from cleaning system 10 through overflow drain tube 34.
While the foregoing system for draining excess liquid and preventing overflow is generally adequate for preventing acid from spilling into the surroundings, a problem has been observed during operation of the system. This problem arises when a portion of the liquid becomes airborne in cleaning system 10. For example, as liquid moves from first volume 20 to second volume 22, some of the liquid is splashed or otherwise projected into the air above second volume 22 at some point during operation of cleaning system 10. Alternatively, when the liquid in cleaning system 10 is heated, the liquid can be head to a rolling boil which may cause the liquid to splash or otherwise be projected into the air above second volume 22. Airborne liquid 40, from either of the two foregoing scenarios, can fall through opening 38 and be prematurely drained from the cleaning system 10 through overflow drain tube 34.
It has been found that during extended operation of cleaning system 10, enough of the liquid becomes airborne and passes through opening 38 to significantly lower the elevation of second volume 22. Eventually, second volume 22 may be left with little or no liquid such that pump 18 begins to draw air through outlet channel 32. In such situations, the entire system is disrupted, often to the extent that semiconductor wafers 16 are dislodged from their position within first reservoir 12. Accordingly, semiconductor wafers 16 may be damaged or, in any event, the system must be shut down for a period of time which leads to inefficiency and increases the cost of the manufacturing process.
FIG. 3 illustrates a cantilevered shielding apparatus 42 that has been developed in response to the foregoing problem of premature drainage of liquid through overflow drain tube 34. Cantilevered shielding apparatus 42 covers opening 38 to prevent airborne liquid 40 from passing therethrough. Cantilevered shielding apparatus 42 is supported by inner wall 28 and is secured thereto by means of a screw 44 or other tightening device. However, cantilevered shielding apparatus 42 has often proved unsatisfactory because it is easily dislodged from inner wall 28. When cantilevered shielding apparatus 42 is displaced or falls out of position, it loses its effectiveness, and airborne liquid 40 begins to be projected into opening 38 once again. In such circumstances, cleaning system 10 is easily disrupted as if no shielding apparatus were present, and semiconductor wafers 16 are again at risk of being damaged.
In view of the foregoing, it would be an advancement in the art to provide a shielding apparatus that prevents entry of airborne liquid into an overflow drain tube while reliably remaining in position relative to the drain tube. It would also be an advancement in the art to provide systems and methods for cleaning semiconductor structures using such a reliable shielding apparatus.